This application claims priority based on Japanese Patent Application No. 2000-099150 filed Mar. 31, 2000 and entitled xe2x80x9cGolf Club Shaft.xe2x80x9d
The present invention relates to a golf club shaft formed of fiber reinforced plastics, and more specifically, to a golf club shaft that maintains a desired flexural rigidity and strength and that has an improved distribution of linear density along the longitudinal shaft axis.
A general golf shaft club has an outside diameter that increases linearly from the tip end to the butt end so that the cross-sectional area of the shaft increases linearly. The distribution of linear density, which is the distribution of the mass of shaft material per unit length, is proportional to the axial variation of the cross-sectional area. Therefore, the linear density increases substantially linearly from the tip end to the butt end.
In relation to the distribution of linear density described above, a number of shafts with the varied distribution of linear density have been devised to improve their swing characteristics.
For example, Japanese Unexamined Patent Application No. Hei 7-163689 discloses a shaft that has at least one balancing weight placed within a cylindrical wall of composite material. The balancing weight is made of a formed sheet and is attached to the inner face of the cylindrical wall. Japanese Patent No. 2622428 discloses a shaft with a bulge. The bulge is produced by cutting sheets of prepregs into a predetermined shape and laminating them to drastically increase the outside and inside diameters of the shaft.
However, the distribution of flexural rigidity EI is limited in these shafts.
In the shaft with a weight (a balancing weight), the specific gravity of the weight is larger than that of carbon fiber reinforced plastics (CFRP). It is likely that the elastic modulus E of the weight is also larger than that of CFRP. Therefore, the flexural rigidity EI and the stress increase at the position where the weight is attached. This lowers the shaft strength.
The shaft with a bulge has the same problem. Provided that the shaft has an outside diameter (d2) and an inside diameter (d1), the second moment of area I of the shaft can be expressed as xcfx80(d24xe2x88x92d14)/64. The second moment of area I is increased either by increasing the outside diameter (d2) or by decreasing the inside diameter (d1). When the elastic modulus E is constant, the value EI will increase as the second moment I increases. More stress is applied on the location where the variation of flexural rigidity EI is greater. This reduces the shaft strength.
One method to improve the flexural rigidity EI is to introduce a separate object at the position on the shaft where second moment of area I should be increased (low Exc3x97high I=good EI). However, this increases the number of parts and manufacturing steps and thus increases manufacturing costs. Accordingly, this method is impractical.
Japanese Unexamined Patent Application No. Hei 6-278216 discloses a process of manufacturing a tubular body including a golf shaft. The process includes winding fibers over a mandrel, impregnating the fibers with a resin, heat curing the resulting material, and pulling out the mandrel. In this process, longitudinal yarns parallel to the longitudinal axis of the tubular body are provided. At least one of an inner layer and a part of an intermediate layer include the longitudinal yarns. A plurality of layers, including a layer which includes the longitudinal yarns, can be manufactured at a time. This process provides a tubular body that is manufactured in one step and that has a desired flexural rigidity EI. The yarns, except for the longitudinal yarns, are wound circumferentially and continuously over the mandrel. The flexural rigidity EI along the length of the tubular body may be varied by changing the winding angle. However, in this process, the distribution of mass of the shaft of the tubular body is not considered.
To solve these problems, it is an objective of the present invention to provide a golf club shaft that maintains a desired flexural rigidity EI and strength and that has an improved distribution of linear density along the longitudinal shaft axis.
The golf shaft club of the invention has a shaft extending between a tip portion and a butt portion. The shaft includes braided yarns of reinforcing fibers that form layers. The layers are hardened with resin. The layers include a varied layer that has yarns positioned substantially symmetrically at predetermined orientation angles relative to the shaft axis. The orientation angles are maximum at a predetermined location and decrease from the predetermined location towards the tip portion and the butt portion.